how to render emission of light of a transparent background
Note : this answer is obsolete due to the comments. I don't delete it as it still produces a result.

Use the compositor.

The setup can be like this :



On the left, the rendered input (here, all is black around, because the world has no material or light set to 0).

The color ramp is here to convert eventual colored parts of your scene to black and white.

Alpha over puts a transparency (the first image slot) over the rendered image, depending on the B&amp;W chanel.